1. Field of the Invention
The present invention relates to a piezoelectric transformer which transforms the amplitude of an alternating voltage by the piezoelectric effect of a piezoelectric material such as a piezoelectric ceramic.
2. Description of the Related Art
A piezoelectric transformer which was designed for a step-up transformer of a high voltage power supply has not been commercialized because of limited properties of a piezoelectric ceramic material such as breaking strength. However, with the advance of high strength piezoelectric ceramics, attention has been recently paid again to the piezoelectric transformer as the step-up transformer for an inverter of a backlight source of the liquid crystal display (LCD) panel installed on a portable information equipment, in the face of the increasing demand for the thin and compact equipment such as a notebook personal computer and a portable terminal.
In that information equipment, the inverter for the LCD is used, for example, as a power supply for lighting a cold cathode fluorescent lamp (CCFL) which is employed as a backlight source. This inverter must be a kind capable of converting a direct current voltage of about 3V to 12V from a battery or the like to a high frequency high voltage of 1 kVrms when starting lighting of the backlighting elements and about 500 Vrms when constantly lighting the backlighting elements and a frequency of about 60 to 80 kHz. An electromagnetic transformer which is used at present for the inverter for the backlight source satisfies the demand for making the equipment thin as a horizontal type transformer using a core of special shape. However, there is a limit to making the electromagnetic transformer small in size and thin because it needs to have a withstand voltage against the voltage as high as several kVrms. Further, winding loss disadvantageously increases and transform efficiency disadvantageously decreases because a thin copper wire is employed to increase the number of turns for stepping up voltage. Besides, the loss disadvantageously occurs which is caused by the material of the core.
The piezoelectric transformer is produced by forming primary (input) electrodes and secondary (output) electrodes on a piezoelectric ceramic material such as lead zirconate titanate (PZT) or piezoelectric crystal material such as lithium niobate. If an alternating voltage having a frequency near the resonance frequency of the piezoelectric transformer is applied to the primary electrodes to mechanically vibrate the piezoelectric transformer, the mechanical vibrations are transformed to a voltage by the piezoelectric effect, which makes it possible to obtain a high voltage from the secondary electrodes in accordance with the impedance ratio between the primary and secondary electrodes. Thus, the piezoelectric transformer can be made smaller in size and thinner than the electromagnetic transformer and can achieve high transform efficiency.
A conventional piezoelectric transformer will next be described with reference to the drawings.
FIG. 31 is a perspective view of a conventional piezoelectric transformer 100. The piezoelectric transformer 100 includes an electrode 104 and an electrode 106 serving as primary (input) electrodes which are formed opposed to each other on almost the left halves of main surfaces of a rectangular plate 102 made of a piezoelectric material perpendicular to the thickness direction thereof, and an electrode 108 serving as a secondary (output) electrode which is formed on one end face of the rectangular plate 102 in the longitudinal direction thereof. If the rectangular plate 102 is made of a piezoelectric ceramic such as lead zirconate titanate (PZT), as indicated by arrows in FIG. 31, the rectangular plate 102 is polarized in advance in the thickness direction thereof on the left half thereof by using the electrodes 104 and 106 and is polarized in advance in the longitudinal direction thereof on the right half thereof by using the electrodes 104, 106 and 108. If an alternating voltage having a frequency near the resonance frequency of the piezoelectric transformer 100 for exciting mechanical vibrations to expand and contract the rectangular plate 102 in the longitudinal direction thereof is applied between the electrodes 104 and 106 (the electrode 106 is a common electrode), the longitudinal extensional vibrations are excited in the piezoelectric transformer 100. These mechanical vibrations are transformed to a voltage by the piezoelectric effect. As a result, it is possible to fetch a high voltage between the electrodes 108 and 106 which serve as the secondary electrodes in accordance with the impedance ratio between the electrodes 104 and 106 serving as the primary electrodes, and the electrodes 108 and 106 serving as the secondary electrodes.
FIG. 32(1) is a side view of the piezoelectric transformer 100 shown in FIG. 31. In FIG. 32(1), arrows indicate the directions in which the rectangular plate 102 is polarized in advance. FIG. 32(2) shows the displacement distribution of the piezoelectric transformer 100 in the longitudinal direction thereof at a certain point of time while extensional vibrations of a half wavelength are generated in the piezoelectric transformer 100 in the longitudinal direction thereof. In FIG. 32(2), the horizontal axis indicates the position in the piezoelectric transformer 100 in the longitudinal direction thereof. The vertical axis indicates the displacement of the piezoelectric transformer 100 in the longitudinal direction thereof caused by the mechanical vibration at a certain instance. On the vertical axis, + direction indicates the right displacement of the piezoelectric transformer 100 in the longitudinal direction thereof and − direction indicates the left displacement thereof in the longitudinal direction thereof. Further, FIG. 32(3) shows the internal stress distribution in the rectangular plate 102 when the piezoelectric transformer 100 has the displacement distribution shown in FIG. 32(2). FIG. 32(4) shows the vibration-induced electric charge distribution when the piezoelectric transformer 100 has the displacement distribution shown in FIG. 32(2). In FIG. 32(3), the horizontal axis indicates the position in the piezoelectric transformer 100 in the longitudinal direction thereof and the vertical axis indicates the magnitude of the internal stress in compression/expansion direction along the longitudinal direction thereof. In FIG. 32(4), the horizontal axis indicates the position in the piezoelectric transformer 100 in the longitudinal direction thereof and the vertical axis indicates the positive/negative polarity and quantity of the electric charges induced by the vibrations. As is obvious from FIGS. 32(3) and 32(4), in the central portion of the rectangular plate 102, that is, in the portion in which the rectangular plate 102 has a vibration displacement of 0, the rectangular plate 102 has the maximum internal stress and the largest quantity of the induced electric charges. Such a piezoelectric transformer in which the mechanical vibrations of a half wavelength are excited, for example, having the displacement distribution shown in FIG. 32(2) is normally referred to as a “piezoelectric transformer having a λ/2 longitudinal extensional vibration mode (where λ indicates one wavelength)”.
Generally, when extremely large strains are caused by the mechanical vibrations in a piezoelectric transformer, the possibility that a piezoelectric transformer will break is high, which leads to a deterioration in reliability. It is, therefore, necessary to hold down the amplitudes of the mechanical vibrations of the piezoelectric transformer as much as possible. Even if the piezoelectric transformer handles high electric power, it is possible to decrease the amplitudes of the mechanical vibrations of the piezoelectric transformer by making the piezoelectric transformer thicker and wider. However, if a space in which the piezoelectric transformer can be arranged is restricted in a system or an equipment into which the piezoelectric transformer is introduced, there is a limit to holding down elastic strains only by its shape.
Furthermore, even if the power which the piezoelectric transformer handles is as low as several watts, the power handled by the piezoelectric transformer per unit volume is large when it is required to further make the piezoelectric transformer small in size, thin and small in height, introducing the piezoelectric transformer into a system such as a portable equipment. As a result, also in this case, as in the case where the piezoelectric transformer handles high power, the piezoelectric transformer which is high in reliability as well as small in size and thin cannot be realized in terms of the mechanical strength thereof.
Moreover, if the rectangular plate of the piezoelectric transformer is made of a piezoelectric ceramic, a part of the piezoelectric transformer in which polarization directions are discontinuous is lower in mechanical strength than a part thereof in which polarization directions are continuous due to the influence of a strain generated while polarizing. In the case of the conventional piezoelectric transformer 100 having a λ/2 longitudinal extensional vibration mode shown in FIG. 31 and FIG. 32(1), a part of the rectangular plate 102 in which a high stress is generated during normal operation (a point P in FIG. 32(3)) almost coincides with a part in which polarization directions are discontinuous (a part near an electrode 108—side end portion of a region of the rectangular plate 102 interposed between the electrodes 104 and 106). Thus, if the power handled by the piezoelectric transformer 100 increases and the amplitudes of the mechanical vibrations increase, then the high stress is generated in the part in which the polarization directions are discontinuous and cracks disadvantageously tend to occur.
Furthermore, even if the rectangular plate 102 is made of piezoelectric monocrystals which does not need the polarizing processing (in this case, arrows in FIGS. 31 and 32(1) indicate the directions of c axis orientations), it is required to change the directions of c axis orientations by laminating elements which are different in the direction of the c axis and conducting a method corresponding to polarization treatment in the case of a piezoelectric ceramic, realizing the piezoelectric transformer having the structure as shown in FIGS. 31 and 32(1). Thus, also in the case of rectangular plate 102 made of piezoelectric monocrystals, as in the case of the rectangular plate 102 made of a piezoelectric ceramic, a part of the rectangular plate 102 in which the directions of the c axis orientations are discontinuous is lower in mechanical strength than a part thereof in which the directions of the c axis orientations are continuous. As a result, if the power handled by the piezoelectric transformer 100 increases and the amplitudes of the mechanical vibrations increases, then the high stress is generated in the part in which the directions of the c axis orientations are discontinuous and cracks disadvantageously tend to occur.
Next, another conventional piezoelectric transformer disclosed in JP Laid-open Patent Publication No. 9-74236 etc. will be described. The piezoelectric transformer, different from the piezoelectric transformer 100 shown in FIG. 31, has a structure in which a part having the highest stress caused by the mechanical vibrations does not coincide with a part in which polarization directions are discontinuous.
FIG. 33 is a perspective view of the piezoelectric transformer 120 having a λ/2 longitudinal extensional vibration mode wherein a part having the maximum stress caused by the mechanical vibrations does not coincide with any part in which the polarization directions are discontinuous. Electrodes 124 and 126 which serve as primary (input) electrodes are formed opposed each other in the thickness direction of the rectangular plate 122 made of a piezoelectric ceramic material at the central portions of two main surfaces thereof perpendicular to the thickness direction thereof. Electrodes 128 and 130 which serve as secondary (output) electrodes are formed opposed each other on two end faces of the rectangular plate 122 in the longitudinal direction thereof. As indicated by arrows in FIG. 33, the rectangular plate 122 is polarized in the thickness direction thereof between the electrodes 124 and 126 serving as the primary electrodes and also is polarized in the longitudinal direction thereof between the primary and secondary electrodes.
FIG. 34(1) is a side view of the piezoelectric transformer 120 shown in FIG. 33. FIG. 34(2) shows the displacement distribution of the piezoelectric transformer 120 in the longitudinal direction thereof at a certain point of time while extensional vibrations of a half wavelength are generated in the piezoelectric transformer 120 in the longitudinal direction. FIG. 34(3) shows the internal stress distribution of the rectangular plate 122 when the piezoelectric transformer 120 has the displacement distribution shown in FIG. 34(2). FIG. 34(4) shows the electric charge distribution induced to the rectangular plate 122 by the vibrations when the piezoelectric transformer 120 has the displacement distribution shown in FIG. 34(2). Arrows shown in FIG. 34(1) indicate polarization directions as those shown in FIG. 33. In FIG. 34(2), the horizontal axis indicates the position in the piezoelectric transformer 120 in the longitudinal direction thereof and the vertical axis indicates the displacement of the piezoelectric transformer 120 in the longitudinal direction thereof caused by a mechanical vibration of the piezoelectric transformer 120 at a certain instance. On the vertical axis, + direction indicates the right displacement of the piezoelectric transformer 120 in the longitudinal direction thereof and − direction indicates the left displacement thereof in the longitudinal direction thereof. In FIG. 34(3), the horizontal axis indicates the position in the piezoelectric transformer 120 in the longitudinal direction thereof and the vertical axis indicates the magnitude of the internal stress in compression/expansion direction along the longitudinal direction thereof. In FIG. 34(4), the horizontal axis indicates the position in the piezoelectric transformer 120 in the longitudinal direction thereof and the vertical axis indicates the positive/negative polarity and quantity of the electric charges induced by the vibrations.
As in the piezoelectric transformer 100 shown in FIG. 31, the λ/2 longitudinal extensional vibration mode is excited in the piezoelectric transformer 120. If an alternating voltage having a frequency near the resonance frequency of the piezoelectric transformer 120 for exciting the mechanical vibrations to expand and contract the rectangular plate 122 in the longitudinal direction thereof is applied between the electrodes 124 and 126 serving as the primary electrodes, with the electrode 126 used as a common electrode, then the mechanical vibrations to expand and contract the rectangular plate 122 in the longitudinal direction thereof which has the displacement distribution shown in FIG. 34(2) was excited on the piezoelectric transformer 120. These mechanical vibrations are transformed to a voltage by the piezoelectric effect. The voltage can be fetched as a high voltage between the electrodes 126 and 128 and between the electrodes 126 and 130 in accordance with the impedance ratio between the primary and secondary electrodes.
As shown in FIGS. 34(1) to 34(4), a part having the highest stress caused by the mechanical vibration (a point P in FIG. 34(3)) does not coincide with any part in which polarization directions are discontinuous (a part near an electrode 128—side end portion of a region of the rectangular plate 122 interposed between the electrodes 124 and 126, and a part near an electrode 130—side end portion of a region of the rectangular plate 122 interposed between the electrodes 124 and 126). Therefore, the piezoelectric transformer 120 has an excellent structure for handling a high power.
However, as the power handled by the piezoelectric transformer 120 per unit volume increases, the amplitudes of the mechanical vibrations increase and the elastic strains increase because the piezoelectric transformer 120 shown in FIG. 34 uses the λ/2 longitudinal extensional vibration mode as in the case of the piezoelectric transformer 100 shown in FIG. 31. In addition, if a space in which the piezoelectric transformer can be arranged is restricted in a system or an equipment into which the piezoelectric transformer 120 is introduced, there is a limit to holding down the elastic strains by its shape.
Furthermore, a method for using the 3λ/2 longitudinal extensional vibration mode is proposed in Japanese Patent No. 2850216 etc. By using this method, the amplitudes of the mechanical vibrations can be decreased, which holds down elastic strains, and the driving frequency can be increased. Thus, the power handled by a piezoelectric transformer for one vibration is reduced and the number of vibrations is increased, which enables the piezoelectric transformer to handle the high power.
The piezoelectric transformer shown in the Japanese Patent No. 2850216 etc. will now be described. FIG. 35 is a perspective view of the piezoelectric transformer 140 having the 3λ/2 longitudinal extensional vibration mode. On a rectangular plate 142 made of piezoelectric ceramic or the like, electrodes 143, 144, 145, 146, 147 and 148 serving as primary (input) electrodes are formed on the two main surfaces of the rectangular plate 142 perpendicular to the thickness direction thereof on a rectangular plate 142 made of piezoelectric ceramic or the like, and an electrode 154 serving as a secondary (output) electrode is formed on one end face of the rectangular plate 142 in the longitudinal direction thereof. Two electrodes of the electrodes 143 and 144, those of the electrodes 145 and 146, and those of the electrodes 147 and 148 are respectively formed to be opposed each other in the thickness direction of the rectangular plate 142. As indicated by arrows shown in FIG. 35, the rectangular plate 142 is polarized in advance in the thickness direction thereof between the primary electrodes using the primary electrodes and is also polarized in advance in the longitudinal direction thereof between the electrodes 147 and 148 serving as the primary electrodes and the electrode 154 serving as the secondary electrode using the electrode 154.
FIG. 36(1) is a side view of the piezoelectric transformer 140 shown in FIG. 35. Arrows shown in FIG. 36(1) indicate polarization directions as those shown in FIG. 35. FIG. 36(2) shows the displacement distribution of the piezoelectric transformer 140 in the longitudinal direction thereof at a certain point of time while extensional vibrations of a 2/3 wavelength are generated in the piezoelectric transformer 140 in the longitudinal direction thereof. FIG. 36(3) shows the internal stress distribution of the rectangular plate 142 when the piezoelectric transformer 140 has the displacement distribution shown in FIG. 36(2). FIG. 36(4) shows the electric charge distribution induced to the rectangular plate 142 by the vibrations when the piezoelectric transformer 140 has the displacement distribution shown in FIG. 36(2). In FIG. 36(2), the horizontal axis indicates the position in the piezoelectric transformer 140 in the longitudinal direction thereof and the vertical axis indicates the displacement of the piezoelectric transformer 140 in the longitudinal direction thereof caused by a mechanical vibration thereof at a certain instance. On the vertical axis, + direction indicates the right displacement of the piezoelectric transformer 140 in the longitudinal direction thereof and − direction indicates the left displacement of the piezoelectric transformer 140 in the longitudinal direction thereof. In FIG. 36(3), the horizontal axis indicates the position in the piezoelectric transformer 140 in the longitudinal direction thereof and the vertical axis indicates the magnitude of internal stress in compression/expansion direction along the longitudinal direction thereof. In FIG. 36(4), the horizontal axis indicates the position in the piezoelectric transformer 140 in the longitudinal direction thereof and the vertical axis indicates the positive/negative polarity and quantity of the electric charges induced by the vibrations.
In the piezoelectric transformer 140, the electrodes 144, 145 and 148 are electrically connected to one another and serve as primary electrodes, and the electrodes 143, 146 and 147 are electrically connected to one another and serve as common electrodes. If an alternating voltage having a frequency near resonance frequency for exciting the mechanical vibrations to expand and contract the rectangular plate 142 in the longitudinal direction thereof is applied between the primary electrodes and the common electrodes of the piezoelectric transformer 140, then the piezoelectric transformer 140 excites the mechanical vibrations to expand and contract the rectangular plate 142 in the longitudinal direction thereof having the displacement distribution shown in FIG. 36(2). The excited mechanical vibrations are transformed to a voltage by the piezoelectric effect. The voltage can be fetched between the electrode 154 serving as the secondary electrode and the common electrodes as a high voltage in accordance with the impedance ratio between the primary and secondary electrodes.
This piezoelectric transformer 140 has the 3λ/2 longitudinal extensional vibration mode. Thus, the amplitudes of the mechanical vibrations can be decreased, which holds down elastic strains, and the driving frequency can be increased. Therefore, the power handled by a piezoelectric transformer for one vibration is reduced and the number of vibrations is increased, which enables the piezoelectric transformer to handle the high power.
However, as in the case of the piezoelectric transformer 100 having the λ/2 longitudinal extensional vibration mode, in the piezoelectric transformer 140 having the 3λ/2 longitudinal extensional vibration mode, a part having the highest stress caused by the mechanical vibrations (point P in FIG. 36(3)) almost coincides with a part in which polarization directions are discontinuous (a part near the electrode 154—side end portion of a region the rectangular plate 142 interposed between the electrodes 147 and 148). Thus, the high stress is generated in the part of the rectangular plate 142 in which the mechanical strength is low and the polarization directions are discontinuous, with the result that cracks disadvantageously tend to occur.
With a view of solving the above-described disadvantages, a piezoelectric transformer will be considered in which the vibrations of the 3λ/2 longitudinal extensional vibration mode are excited and which has the same structure as the piezoelectric transformer 120 (see FIG. 33) having the λ/2 longitudinal extensional vibration mode wherein the part having the highest stress caused by the mechanical vibrations does not coincide with the part in which the polarization directions are discontinuous. FIG. 37(1) is a side view of the piezoelectric transformer 120 shown in FIG. 33. FIG. 37(2) shows the electric charge distribution induced to the rectangular plate 122 at a certain point of time when the vibrations of the 3λ/2 longitudinal extensional vibration mode are excited. FIG. 37(3) shows the displacement distribution of the piezoelectric transformer 120 caused by the mechanical vibration in the longitudinal direction thereof when the electric charge distribution shown in FIG. 37(2) is induced to the rectangular plate 122. In FIG. 37(2), the horizontal axis indicates the position in the piezoelectric transformer 120 in the longitudinal direction thereof and the vertical axis indicates the positive/negative polarity and quantity of the electric charges induced by the vibration. In FIG. 37(3), the horizontal axis indicates the position in the piezoelectric transformer 120 in the longitudinal direction thereof and the vertical axis indicates the displacement of the piezoelectric transformer 120 caused by the mechanical vibrations thereof in the longitudinal direction thereof. On the vertical axis, + direction indicates the right displacement of the piezoelectric transformer 120 in the longitudinal direction thereof and − direction indicates the left displacement of the piezoelectric transformer 120 in the longitudinal direction thereof.
In this case, it is possible to decrease the amplitudes of the mechanical vibrations of the piezoelectric transformer 120 and to hold down elastic strains because the piezoelectric transformer 120 has the 3λ/2 longitudinal extensional vibration mode. Besides, for this piezoelectric transformer 120, the problem that cracks tend to occur can be solved because any part having the highest stress caused by the mechanical vibrations does not coincide with any part in which polarization directions are discontinuous.
However, this piezoelectric transformer also has the following disadvantages. Generally, a piezoelectric transformer transforms electrical energy input into primary electrodes to mechanical energy. The mechanical energy is fetched from a secondary electrode of the piezoelectric transformer as electrical energy. If an effective electromechanical coupling factor keff which shows the rate at which the piezoelectric member can transform electrical energy applied to the primary electrodes to mechanical energy is high, then the electrical energy can be transformed to the mechanical energy at high rate and the piezoelectric transforms can handle high power per volume. In a λ/2 longitudinal extensional vibration mode, if the thickness and the width of the rectangular plate of the piezoelectric transformer are the same, the larger the sum of the lengths of the primary electrodes of the piezoelectric transformer on the main surface is, the higher the effective electromechanical coupling factor keff is.
However, in the piezoelectric transformer 120 having the 3λ/2 longitudinal extensional vibration mode as shown in FIG. 37(1), as shown in FIG. 37(3), the maximum amplitude of the mechanical vibrations in the piezoelectric transformer 120 when each length of the electrodes 124 and 126 serving as the primary electrodes is set large enough to exceed the point at which the polarity of the induced electric charges changes as shown by solid line is lowered than that when each length of primary electrodes does not exceed the point at which the polarity of the induced electric charges changes (indicated by one-dot chain lines in FIG. 37(3)), by as much as electric charge cancellation quantities (indicated shaded portions shown in FIG. 37(2)). This means that the effective electromechanical coupling factor keff which shows the rate of transforming the electrical energy to the mechanical energy at the primary electrodes decreases. As a result, there is the problem that the power handled by the piezoelectric transformer decreases.
Meanwhile, if each length of the electrodes 124 and 126 serving as the primary electrodes of the piezoelectric transformer 120 is set not to exceed the point at which the polarity of the electric charges induced by the vibrations changes as shown in FIG. 38(1) so that electric charges induced by the vibrations do not cancel one another, no electric charge cancellation occurs as shown in FIG. 38(2) and the maximum amplitude of the mechanical vibrations in the piezoelectric transformer 120 is higher than that indicated by a solid line in FIG. 37(3) as shown in FIG. 38(3). However, the lengths of the electrodes 124 and 126 serving as the primary electrodes are limited to the lengths of a half wavelength with respect to that of an elastic wave of 3/2 wavelength excited to the rectangular plate 122, which means that it is impossible to set the effective electromechanical coupling factor keff high. As a result, the power handled by the piezoelectric transformer 120 is disadvantageously limited.
Moreover, a piezoelectric transformer having a low output impedance is desired. For such a piezoelectric transformer, the current supply ability is high for supplying current to a load such as a cold cathode fluorescent lamp connected to the piezoelectric transformer. A piezoelectric transformer capable of setting a step-up ratio high is also desired. Further, it is desired to realize a piezoelectric transformer having a high driving efficiency. A piezoelectric transformer capable of handling the large power with small strains is also desired. It is further desired to be able to decrease the number of manufacturing steps of manufacturing a piezoelectric transformer and time required to manufacture the piezoelectric transformer. It is also desired to be able to provide a supporter capable of supporting and fixing a piezoelectric transformer without obstructing the vibrations of the piezoelectric transformer and capable of ensuring to connect electrically the input and output electrodes of the piezoelectric transformer to exterior through one's terminals or the like.